Roscovitine triggers apoptosis in B-cell chronic lymphocytic leukemia cells with similar efficiency as combinations of conventional purine analogs with cyclophosphamide.

B-cell chronic lymphocytic leukemia (CLL) is characterized by an accumulation in peripheral blood of many long-lived lymphocytes that do not die because of the deregulation of apoptosis. Most CLL cells are quiescent, and therefore the leukemic lymphocytes are resistant to conventional chemotherapy. The aim of this study was to evaluate in vitro the chemosensitivity of CLL cells to cladribine or fludarabine used alone or in combinations with mafosfamide (Mf; the active form of cyclophosphamide) as well as to roscovitine, a potent inhibitor of cyclin-dependent kinases with proapoptotic potential. The results of flow cytometry revealed that tested agents differentially reduced the viability of leukemic cells. Interestingly, roscovitine exerts a similar cytotoxic effect as the combinations of the used purine analogs with Mf, but with other kinetics. Roscovitine kills leukemic cells after a much shorter exposure time. Immunoblotting analysis showed that the reduction of the number of living cells coincides with marked changes of the balance between pro- and antiapoptotic factors. The latter were markedly reduced. The activation of proapoptotic proteins became evident especially after exposure of cells to roscovitine alone or to combinations of purine analogs and Mf. Furthermore, exposure of CLL cells to tested drugs degraded p27(KIP1) protein. Our findings demonstrate that roscovitine alone significantly reduces the number of viable CLL cells by inducing them to undergo apoptosis, and it acts earlier than clinically applied combinations of purine analogs with Mf/cyclophosphamide. These results confirm the high efficacy of roscovitine against CLL cells.